Investments - Schedule of Fixed Maturities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fixed Maturities Available for Sale, Amortized Cost
Due in one year or less	$ 85,822
Due after one year through five years	648,451
Due after five years through ten years	1,686,684
Due after ten years through twenty years	5,394,107
Due after twenty years	8,399,480
Mortgage-backed and asset-backed securities	201,232
Cost or Amortized Cost	16,415,776	$ 15,753,471
Fixed Maturities Available for Sale, Fair Value
Due in one year or less	86,840
Due after one year through five years	699,017
Due after five years through ten years	1,937,987
Due after ten years through twenty years	6,500,168
Due after twenty years	9,460,036
Mortgage-backed and asset-backed securities	223,099
Fair Value	$ 18,907,147	$ 16,297,932